There have been few major changes to the manner in which acoustic guitars are made since Martin guitars of the USA built the first steel string acoustic guitars in the 1850s.
It could be said, in a simplified version of events, that the steel string acoustic guitar is an American adaptation of the Spanish ‘classical’ gut string acoustic guitar, but with steel strings.
The advent of the steel string guitar also heralded substantial changes in the method of manufacture.
Generally Spanish built guitars were assembled ‘as a whole’; that is, the neck component was included with the assembly of the resonant cavity.
By contrast the Martin company decided to build the body or resonant cavity of the instrument and to attach the neck by means of what is called a wedged dovetail joint.
The major components of the body or resonant cavity of an acoustic guitar include:
a front panel, often referred to as the top, face or soundboard;
a back panel; and
a side wall, extending between the peripheries of the front and back anels.
To the top of the neck component, or arm of the guitar, is affixed a ‘fingerboard’, which allows the player to ‘fret’ or play notes.
The ‘headstock’ or ‘peg head’ of the guitar is at an end of the neck opposite to the end joined to the body and provides an area where string tension adjustment devices are affixed. These devices are known as the ‘tuning keys’ or ‘machine heads’.
Between the headstock and the end of the fingerboard is the ‘nut’. The strings rest on the ‘nut’ at the peg head end of the guitar and are affixed to the face or top of the guitar by the ‘bridge’.
The ‘bridge’ is a wooden component, which is attached/adhered to the face or soundboard of the instrument and which provides the anchor point for the string ends.
The ‘saddle’ is ‘let in’ to a slot in the bridge and is usually kept in place by string pressure. The saddle is positioned to give the appropriate string length in order that the strings ‘intonate’ or resonate at the desired frequencies. The strings oscillate when played or strummed between the ‘nut’ and the ‘saddle’.
The Current Assembly Method
Timber is sawn and sanded into thin sections, the back and top generally to 2.8 to 3.2 mm, and the sides: 1.8 to 2.2 mm.
Since the top or soundboard, is thin and would not be able to withstand the compressive force of the strings, which at concert pitch where the ‘A’ string is tuned to 440 KHz, is around 70 kg for the collective 6 strings fitted to a ‘normal’ acoustic guitar, the soundboard needs to be ‘braced’ or ‘strutted’.
Longitudinal sections of timber are adhered to add strength. These pieces of timber are usually around 6 to 8 mm in width and 10 to 15 mm in height and are adhered onto the inner surface of the soundboard in the shape of an ‘X’, and are therefore often referred to as ‘X’ braces.
The back of the guitar is usually strengthened by four pieces of timber of similar or larger dimension to those adhered to the top, running across, from one side to the other of the instrument. These pieces are referred to as ‘back braces’.
Since the sides are 1.8 to 2.2 mm in thickness and need to be adhered to the top and back, the gluing surface needs to be increased. This is achieved by the addition of ‘kerfing’. Kerfing is a length of timber equal to the length of the sides, generally triangular in cross section, 5 to 10 mm in height and 5 to 10 mm in width. In order that the kerfing is able to conform to the curvaceous shape of the sides, it is cut almost through, by means of a saw, at regular intervals of around 10 to 15 mm. Since the space left in timber by a saw blade is called the ‘kerf’, it follows that this ‘component’ has become known as ‘kerfing’.
The kerfing is clamped and adhered to the top and bottom of the sides to increase the surface area in anticipation of the adhesion of the top and the back.
Kerfing is extremely difficult to adhere exactly flush to the sides of the guitar and therefore there is generally an additional operation involved to mill or sand the sides and top of the kerfing to the same level.
The sides of an acoustic guitar are steam bent to the required and typical shape by the use of a heated press (which emulates the finished shape of the guitar) or heating irons, in conjunction with water. In some instances the timber, which will comprise the sides, is soaked in a bath and in other instances it is simply ‘wetted down’ with a sponge.
The sides, along with the kerfing, are clamped into jigs or ‘forms’ that match the top profile, plan view or shape of the guitar.
Once the sides are milled or sanded to an exact shape so that the top and bottoms of the sides are ‘flush’ with the kerfing, the top and back are adhered. The braces or struts on the face and back are trimmed ‘by hand’ to fit in, or to be ‘housed’, or to be flush with the kerfing.
The top and the back components are generally cut to a similar shape to the top profile or plan view of the body, but around 2 to 5 mm oversize or overlapping to ensure fitment. Later, the back and top are ‘trimmed’ to be flush with the sides. This is done as it is unlikely that the thin sections of timber that comprise the sides will exactly conform to the jigs or ‘forms’ into which the sides are temporarily fitted.
The ‘body jigs’ or ‘forms’ are removed once the face and back have been adhered, as the body or resonant cavity will then have its own ‘structural tenacity’.
Internal and at the front of the body, a timber block is adhered to accept the fitting of the neck or arm component.
The methods for neck fitment vary: some makers and manufacturers use a method known as a ‘dovetail’ joint, which is well known in the wood working industry, others affix it by the use of wooden dowels, a method also well known in the woodworking industry and some affix the neck by the use of nuts and bolts.
The neck needs to be attached with great accuracy otherwise the instrument may not be easily played.
Difficulties arise in maintaining the accuracy of the resonant cavity or body, considering that:
the body is comprised of thin sections (1.8 to 2.2 mm) of timber, which therefore do not have integral structural tenacity;
the kerfing needs to be machined to conform to the top and bottom of the sides yet the sides are difficult to clamp or secure, and do not readily provide a clear datum from which they can be machined;
the thin sections of timber and manner of construction allow the timber to be prone to climatic variations, which in turn jeopardises accurate assembly.
Various makers and manufacturers have various equipment and methods of fitting the neck. It is generally a major piece of equipment and the process is often time consuming requiring considerable expertise to perform.
The bridge and ‘saddle’ assembly of the guitar is and needs to be 10 to 15 mm in height in order that the strings are able to exert enough pressure on the face to ensure good sound transference and to ensure that the strings are a sufficient distance from the face to enable the instrument to be played.
This height requirement means that the neck is ‘set’, attached or affixed at an angle back of a few degrees as observed in a side profile view of the instrument.
The obtuse angle makes the manufacturing process complex, and the need for manual adjustment considerable. Such adjustment usually occurs at many stages of production.
Since the advent of CNC routers it has been possible to produce components for musical instruments more accurately and yet, other than the efficiency of individual component manufacture, the improvement in quality and efficiency of instrument assembly has been minimal. Thus, there remains a need for methods and apparatus which improve on conventional techniques.